The present invention relates to a system for generating blue-green light. More particularly, the invention employs the phenomenon of frequency doubling in an optical fiber to convert the infrared output of a semiconductor laser to blue-green light having a wavelength corresponding to a Fraunhofer line.
There are many applications where it is desirable to communicate through sea water, as for example, with submarines and underwater remote piloted vehicles. Because sea water is most transparent to the blue-green region of the electromagnetic spectrum, light falling within this region can be used as a communication medium. Generally, blue-green light is light having a wavelength from 450-550 nm, and blue-green light having a wavelength of about 490 nm has optimal transmissibility through sea water.
A significant problem associated with using blue-green light as a communication channel through sea-water is that sunlight contains spectral components that compete with blue-green light and appears as background noise in the channel. The intensity of sunlight, and hence the degree of noise, varies with the wavelength of the light generated by the sun. For example, sunlight has a maximum intensity at approximately 500 nm, approximately in the middle of the blue-green portion of the electromagnetic spectrum, but has a relative minimum intensity at 486.13 nm. The wavelengths of sunlight at this and other relative intensity minima are referred to as Fraunhofer lines. Fraunhofer lines represent absorption lines in sunlight due to the cooler outer layers of the sun's atmosphere. These Fraunhofer lines are well understood and are catalogued in the National Solar Observatory Atlas No. 1, edited by R. Kurucz, et al. Though Fraunhofer lines of various spectral widths and depths occur throughout the visible spectrum, the line due to hydrogen absorption at 486.13 nm (known as the Hydrogen-Beta line) is among the broadest and deepest. Other lines of interest in the blue-green spectral region occur at 489.1 nm, 495.7 nm, 516.7 nm, 517.3 nm, and 518.4 nm. Thus, it may be appreciated that the signal-to-noise ratio of blue-green light may be increased by using blue-green light corresponding to a Fraunhofer line, and the transmissibility of such light through sea water may be maximized with blue-green light having a wavelength of about 486.13 nm.
U.S. Pat. No. 5,067,134 describes a device for generating blue laser light using a process known as upconversion in an optical fiber. In this process, a glass fiber is doped with an element that will absorb two or more pump photons and re-emit light at a wavelength that is not exactly half of the original wavelength. The device described in the '134 patent includes a semiconductor laser which emits a light beam having a wavelength of 650 nm and a power of 10 mW which is provided to a resonator. The resonator comprises a glass fiber optically coupled between first and second mirrors. The first mirror is transparent to light issuing from the laser. Light propagating through the first mirror is provided to an optical fiber and directed to the second mirror which exhibits limited transparency (approximately 5%) to the blue light generated in the glass fiber. The light beam issued by the resonator has a wavelength of 450 nm and a continuous power of maximally 0.5 mW. However, the wavelength at which this device is designed to operate is significantly lower (i.e., at a higher frequency) than the Fraunhofer line at 486 nm.
U.S. Pat. No. 5,050,233, "Miniature Blue-Green Laser Source Using Second-Harmonic Generation," describes a system and method for generating coherent blue-green light having a wavelength in the range of 490-500 nm. The system employs a diode laser which provides a beam having a wavelength in the range of 980-1000 nm to o an optical resonator which includes a nonlinear crystal of essentially KTP. The crystal produces blue-green radiation by non-critically phase-matched second-harmonic generation of the beam. The beam has a preferred wavelength of 994 nm for generating 497 nm radiation. The frequency of the laser is preferably matched and locked to that of the optical resonator. Since this system requires that the beam propagate at a fixed angle with respect to the optical axis of the crystal (the "Phase Matched" condition), systems of this type are especially sensitive to optical misalignment due to vibration. By necessity, the non-linear conversion process in the KTP crystal requires a very strong electric field which is attained by employing a very high intensity beam. Consequently, these systems require special design considerations to avoid heat damage.
Therefore, a need exists for a rugged, easily manufactured system which generates blue-green light having a wavelength in the range of 450-550 nm and substantially corresponding to a solar Fraunhofer line. More particularly, there is a need for a system for generating an optical signal having a wavelength corresponding to the Hydrogen-Beta Fraunhofer line at 486.13 nm.